Optical objective comprising a meniscus doublet and meniscus triplet between two positive components



SEARCH ROOM r x i ig C w Sept. 23, 1952 w. SCHADE 2,611,295

OPTICAL OBJECTIVE COMPRISING A MENISCUS DOUBLET AND MENISCUS TRIPLET BETWEEN TWO POSITIVE COMPONENTS a Filed June 14, 1951 7 Fig. 2

Willy Schade BY Mi INVEN TOR.

Patented Sept. 23, 1952 SEARC-i iota OPTICAL OBJECTIVE COMPRISING A MENISCUS DOUBLET AND MENISCUS TRIPLET BETWEEN TWO POSITIVE COMPONENTS Willy Schade, Rochester, N. Y., asslgnor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application June 14, 1951, Serial No. 231,624

2 Claims. 1

This invention relates to photographic objectives. 1

The object of the invention is to provide a highly corrected objective particularly suitable for use in cinephotography at apertures up to about 171.4.

The main advantage of the present lens over former lenses of f/1.5 or f/1.4 aperture and equivalent coverage is the highly improved correction of oblique spherical aberration commonly known as rim ray correction. Another advantage in comparison to some highly corrected known lenses of this type is the increased diaphragm space between the components permitting greater convenience in mounting a diaphragm and greater freedom in the adjustment of the spacing when the lens is finally assembled during manufacture.

A type of lens which has been found superior for use as a high aperture cinephotographic objective because of its comparatively long back focal distance and the high degree of correction of which it is capable consists of two compound negative meniscus components concave toward each other and axially aligned between two positive components. It is usual for the front positive component to be weakly concave toward the rear, that is its rear surface is concave with a radius of curvature greater than F wher F is the focal length of the objective, and for the rear component to be biconvex, a plane surface being considered as the limiting form of convex surfacehaving an infinite. radius of curvature, when this type lens is adapted for cinephotography requiring a high aperture and a moderate field coverage of about :15. It is also usual for the concave surfaces of the negative components to be unequal in curvature, that of the third component being the less strongly curved.

As is customary, the front of the objective here and in the appended claims is defined as the end of the objective facing the longer conjugate (object or image) According to the present invention an objective of this known type is made up in which the second component (counting from the front) consists of a meniscus positive element cemented to the front of a meniscus negative element, the cemented surface having a radius of curvature between l.=' and 2F where F is the focal length of the objective and having negative power numerically between 0.01 and 0.05 times the power of the whole objective andin which the third component consists of a positive element cemented between two negative elements of which 2 the rear element has a refractive index between 0.1 and 0.2 lower than the index of the positive element and is separated therefrom by a cemented surface having a radius of curvature between 0.3F and 0.555. The large index difference at this surface has a strong corrective effect on the lower rim rays. The front cemented surface of this third component is much less strongly curved and has a radius of curvature numerically between 0.8F and inclusive. Because of the comparatively weak curvature and comparatively small angle of incidence of the image-forming rays, the index difference at this surface is much less critical than that at the rear cemented surface. In fact, this cemented surface is included in the lens structure primarily for color correction and has only a very minor effect on monochromatic aberrations. The dispersive index of the front element of this triplet component is preferably less than 0.75 times that of the positive element for complete color correction and may be as low as available materials permit, namely about 0.4 times that of the positive element. Its refractive index is determined by two considerations not fundamental to the invention, first that only a limited range of refractive indices (about 1.60 to about 1.85) is available in suitable high-dispersion materials and second that it is convenient (but not essential) in the design of the lens (as is true in optical systems generally) to have a substantially zero index-difmore birefringent than sapphire, a crystal which has been successfully used as an optical material.

The front component is preferably a simple element of glass having a refractive index higher than 1.80 and a dioptric power between 0.4 and 1.0 times that of the whole objective. I find that a high index in this position is very helpful in correcting the barrel distortion which tends to be present in this type of lens.

The rear component preferably has a dioptric power between 0.7 and 1.4 times that of the whole objective. This component is conven iently a simple positive element, but may be made up as a cemented doublet in known manner without departing from the spirit of the invention. In a highly preferred form of the invention, a

highly corrected objective is made up of a negative doublet and a negative triplet aligned between two positive singlets, in which the radii of curvature R and the refractive indices N. each numbered by subscripts from front to rear, are within the limits listed as follows:

and in which the thickness of each negative element is between"0.01F- and 0.06! and" that of each positive'element is between 0.051 and 0.25F, and in which the central space is between 0.2F and -0.35F and each. of the other two spaces is between zero and 0.05F inclusive.

convenience:

[E'=100 mm. 171.4]

Lens N V I Radii 'Ehicknesses mm. mm. 1 1.880 41.1 R1 96 2 h: 8.73 R; =-+303 s1= 0.39 2 1.697 58.1 R: 41 5 :14. 76 3 1.673 32.2 R4 9O 2 h- 3.13 Rs 28 5 sz=27.02 L- 1, 751 27. 7 R4 43 8 h- 3:96 5 1. 755 47. 2 R1 =+531 5 17. 70 6 1.605 37.9 R: 41 3 is 3.95 R0 62 3 si= 0.39 1 1.7.45 45. 8 Rio-+118. t1=l7. 31

Ru -ZOI BF=55.9

In this table the lens elements as numbered from front to rear are designated in the first column, the corresponding refractive indices N for the D line of the spectrum and the convent'ional dispersive indices V are given in the next two columns. The radii -R of the lens surfaces. the thicknesses t of the lens elements and the spaces s between components, each numbered by subscripts from front to rear, are given in the last two columns. as is also the back focal distame BF, for an equivalent focal length-F of 100 mm. It is commonly understood that data for longer or shorter focal lengths are obtained by multiplying all the values in the last two columns-of the table by a constant greater or less-thanone, respectively.

It will be directly evident from the above table that this example embodies all the features of the invention hereinbefore described.

The spherical aberrations S. A. and Coma (defined as: Yi/sin Ue-F-S. A., where Y1 is the distance between the axis and a designated incident ray parallel to the axis and U: is the angle which the same ray makes with the axis after traversing'the lens system) have been computed for the "a" spectral line as follows:

8. A. Coma showing very small zonal aberration. The axial color, paraxially computed. is -0.09 and the lateral color at 10 is +0.006 from the D line to the a line.

The distortion AY, the primary or tangential curvature AF and the secondary or sagittal curvature AF" have been computed in the customary manner and have values. as follows:

for the D spectral line. The symbols All}; AK, and AF" are used because; they designate increments in ray height (Y) and focal length (F) of an actual image point as compared with a theoretically ideal image point. The Petzval sum is +0.002-2.

Four oblique rays at 10 and four at 14 have been computed in addition to the principal rays and were all found to strike the focal plane within +0.083 and -0.173 mm. of the principal ray.---showing a very high degree of correction.

I claim:

1. A photographic objective consisting of two positive components and two compound negative meniscus components concave toward each other spaced between and axially aligned with the two positive components, in which each positive element as hereinafter enumerated has a refractive index between 1.65 and 1.95, in which the'front component is weakly concave toward the rear I and consists of a single positive lens element, in which the second component consists of a meniscus positive element cementedto the front of a meniscus negative element.) the cemented surface having a radius of curvature between 0.51 and 2F and negative power numerically between 0.01? and 0.051? where F is the focal length and P the power of the objective as a whole, in which the third component consists of a positive element cemented between two negative elements of which the rear negative element has a refractive index 1 between 0.1 and 0.2 lower than that of the posifourth component is biconvex and includes one positiveielement:

SEARCH ROOM and in which the thickness of each negative element is between 0.01F and 0.06F and that of each positive element is between 0.05F and 0.25F, and in which the central space is between 02F and 0.35F and each of the other two spaces is between zero and 0.051 inclusive, where F is the focal length of the objective.

WILLY SCHADE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS I Number Name Date 1,779,257 Lee Oct. 21', 1930 2,186,621 Bertele Jan. 9, 1940 2,336,207 Aklin Dec. 7, 1943 FOREIGN PATENTS Number Country Date 470,522 Great Britain Aug. 17, 1937 533,844 Great Britain June 8, 1943 

